Medical Implants Including Iridium Oxide

ABSTRACT

A medical implant includes iridium oxide. The iridium oxide has a plurality of Ir—O σ bonds and a plurality of Ir═O σ bonds. The iridium oxide has a ratio of the Ir—O σ bonds to the Ir═O π bonds that is greater than 1.3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 61/119,646, filed on Dec. 3, 2008, the entire contents of which are hereby incorporated by reference

TECHNICAL FIELD

This invention relates to medical implants including iridium oxide, and more particularly to endoprostheses including iridium oxide.

BACKGROUND

A medical implant can replace, support, or act as a missing biological structure. Some examples of medical implants can include orthopedic implants; bioscaffolding; endoprostheses such as stents, covered stents, and stent-grafts; bone screws; and aneurism coils. Some medical implants are designed to erode under physiological conditions.

Medical endoprostheses can, for example, be used in various passageways in a body, such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprostheses include stents, covered stents, and stent-grafts.

Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, For example, so that it can contact the walls of the lumen.

The expansion mechanism can include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.

In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.

SUMMARY

A medical implant is described that includes iridium oxide having a plurality of Ir—O σ bonds and a plurality of Ir═O π bonds, the ratio of the Ir—O σ bonds to the Ir═O π bonds being greater than 1.3. In some embodiments, the ratio of Ir—O σ bonds to Ir═O π bonds is greater than 1.45. In some embodiments, the ratio is less than 3. For example, the ratio can be between 1.6 and 2.1. The iridium oxide can have a plurality of Ir atoms having a mean valance state of less than 3.4+.

The iridium oxide can have a morphology of defined grains with an aspect ratio of about 5:1 or more.

The iridium oxide can exhibit a charge of at least 0.0060 Coul/cm2 in a cyclic voltammetry potentiodynamic electrochemical measurement having a sweep rate no more than 50 mV/s. In some embodiments, the iridium oxide can have less than 15% atomic carbon.

The iridium oxide can define an outer surface of the medical implant. In some embodiments, the medical implant includes a base metal and the iridium oxide coats the base metal. The base metal can be selected from the group of stainless steels, stainless steels enhanced with radiopaque elements, nickel-titanium alloys, cobalt alloys, titanium and titanium alloys, platinum and platinum alloys, niobium and niobium alloys, tantalum and tantalum alloys, and combinations thereof.

The medical implant can include an electrode, the electrode comprising the iridium oxide.

The medical implant is, in some embodiments, an endoprosthesis (e.g., a stent).

In some aspects, medical implant is described that includes iridium oxide having a plurality of Ir atoms having a mean valance state of less than 3.4+.

In some embodiments, the Ir atoms having a mean valance state of between 3.20+ and 3.35+. In some embodiments, the iridium oxide has a plurality of Ir—O σ bonds and a plurality of Ir═O π bonds, the iridium oxide having a ratio of the Ir—O σ bonds to the Ir═O π bonds that is between 1.45 and 3.0. In some embodiments, the medical device includes a base metal. The iridium oxide can coat the base metal and defining an outer surface of the medical implant. The base metal can be selected from the group of stainless steels, stainless steels enhanced with radiopaque elements, nickel-titanium alloys, cobalt alloys, titanium and titanium alloys, platinum and platinum alloys, niobium and niobium alloys, tantalum and tantalum alloys, and combinations thereof. The medical implant can be an electrode for pacing or neural stimulation. In other embodiments, the medical implant is an endoprosthesis.

In some aspects, method for making a medical implant is described. The method includes placing a medical implant or precursor thereof in a physical vapor deposition processing chamber and depositing iridium oxide on the medical implant or precursor thereof using a power of at least 750 watts and a total pressure of between 3 mTorr and 7 mTorr. In some embodiments, the process can use an oxygen partial pressure of between 91% and 100%,

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A-1C are longitudinal cross-sectional views illustrating delivery of a stent in a collapsed state, expansion of the stent, and deployment of the stent.

FIG. 2 is a perspective view of a stent.

FIGS. 3A and 3B are IROX surface morphologies produced by PVD processes.

FIGS. 3C and 3D are IROX surface morphologies produced by thermal decomposition processes.

FIGS. 4A-4D are Cyclic Voltammetry Scans of three different IROX surfaces.

FIG. 5A depicts an iridium 4f core level spectra of the three different IROX surfaces.

FIG. 5B depicts an Oxygen 1s core level spectra of the three different IROX surfaces.

FIGS. 6A-6C depict deconvoluted Oxygen 1s XPS Core level spectra of the three different IROX surfaces.

FIGS. 7A-7C depict deconvoluted Carbon 1s core level spectra of the three different IROX surfaces.

FIG. 8A depicts inverted cylindrical physical vapor deposition processing chamber.

FIG. 8B depicts a second type of physical vapor deposition processing chamber.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1A-1C, a stent 20 is placed over a balloon 12 carried near a distal end of a catheter 14, and is directed through the lumen 16 (FIG. 1A) until the portion carrying the balloon and stent reaches the region of an occlusion 18. The stent 20 is then radially expanded by inflating the balloon 12 and compressed against the vessel wall with the result that occlusion 18 is compressed, and the vessel wall surrounding it undergoes a radial expansion (FIG. 1B). The pressure is then released from the balloon and the catheter is withdrawn from the vessel (FIG. 1C).

Referring to FIG. 2, stent 20 can have the form of a tubular member defined by a plurality of struts, which include a plurality of bands 22 and a plurality of connectors 24 that extend between and connect adjacent bands. During use, bands 22 can be expanded from an initial, small diameter to a larger diameter to contact the outer diameter of stent 20 against a wall of a vessel, thereby maintaining the patency of the vessel. Connectors 24 can provide the stent 20 with flexibility and conformability that allow the stent to adapt to the contours of the vessel.

Stent 20 can include a stent body formed of a base metal and include an iridium oxide (IROX) coating. For example, IROX coatings are depicted in FIGS. 3A-3D. In some embodiments, the IROX coating is on only one side, e.g. the abluminal side. In other embodiments, the IROX coating can cover all of the sides of the stent body. In some embodiments, the stent can include a second coating, such as a drug-eluting coating. In some embodiments, the second coating can be on only one side, e.g., the abluminal side. In other embodiments, the IROX coating defines an outer surface of the stent and the stent does not include any additional exterior coatings over the IROX coating.

IROX is an electrically conductive, valence-changing metal oxide that crystallizes in the rutile structure. The iridium in IROX can either have a valence state of Ir³⁺ and/or Ir⁴⁺. IROX can reduce restenosis through the catalytic reduction of hydrogen peroxide and other precursors to smooth muscle cell proliferation. IROX can also encourage endothelial growth to enhance endothelialization of the stent. When a stent is introduced into a biological environment (e.g., in vivo), one of the initial responses of the human body to the implantation of a stent, particularly into the blood vessels, is the activation of leukocytes, white blood cells which are one of the constituent elements of the circulating blood system. This activation causes a release of reactive oxygen compound production. One of the species released in this process is hydrogen peroxide, H₂O₂, which is released by neutrophil granulocytes, which constitute one of the many types of leukocytes. The presence of H₂O₂ may increase proliferation of smooth muscle cells and compromise endothelial cell function, stimulating the expression of surface binding proteins which enhance the attachment of more inflammatory cells. IROX can catalytically reduce hydrogen peroxide. IROX is also discussed further in Alt, U.S. Pat. No. 5,980,566. IROX can also facilitate the adhesion of coatings onto an implant. For example, the presence of an IROX coating on a base metal of a stent can prevent the delamination of overlaying polymer coating (e.g., a drug-eluting coating).

The relative amounts of Ir³⁺ and Ir⁴⁺ can impact the catalytic efficiency of the IROX in reducing H₂O₂. The IROX can have a mean valance state of less than 3.4+, meaning that at least 60% of the Iridium atoms present in the IROX have a valance state of 3+. In some embodiments, the IROX can have a mean valance state of between 3.20+ and 3.35+ (e.g., about 3.3+). In addition to other possible reaction mechanisms, IROX can decompose H₂O₂ using the following mechanism, where the H₂O₂ acts as an oxidant and takes electrons from the Ir³⁺ and to convert the Ir³⁺ into Ir⁴⁺. This proposed reaction mechanism indicates that the presence of Ir³⁺ improves the catalytic efficiency of IROX having a lower mean valance state.

H₂O₂+Ir³⁺→Ir⁴⁺+H₂O

3H₂O₂+2IrO(OH)→2IrO₂+4H₂O

IROX includes a plurality of Ir—O σ bonds (single bonds) and a plurality of Ir═O π bonds (double bonds). The ratio of Ir—O σ bonds to Ir═O π bonds (“σ:π ratio”) in IROX can impact the catalytic efficiency of the of the IROX in reducing H₂O₂. The IROX can have a σ:π ratio of greater than 1.3. In some embodiments, the σ:π ratio can be between 1.3 and 3. In some embodiments, the σ:π ratio can be greater than 1.45. More specifically, the σ:π ratio can be between 1.6 and 2.1.

Iridium oxide can form many different chemical structures, even for a given valance state. For example, Ir³⁺ can form Ir(OH)₃, IrO(OH), and Ir₂O₃. Both IrO(OH) and Ir₂O₃ have one Ir—O σ bonds and one Ir═O π bond per iridium atom, while Ir(OH)₃ has 3 Ir—O σ bonds per iridium atom. IROX having Ir⁴⁺ can form IrO₂ and IrO(OH)₂. IrO₂ has two Ir═O π bonds per Iridium atom and IrO(OH)₂ has two Ir—O σ bonds and one Ir═O π bond per iridium atom. Other more complex Iridium oxide chemical structures are also possible. The different chemical structures also can have various oxygen to iridium ratios. Methods of determining the σ:π ratio for IROX samples and the relative amounts of Ir³⁺ and Ir⁴⁺ are discussed below in the Examples section.

The IROX can have a surface morphology of defined grains with an aspect ratio of about 5:1 or more. In some embodiments, the IROX coating can have an S_(dr) of about 100 or greater. The IROX coating can have the types of morphologies described in application U.S. Ser. No. 11/752,772, filed May 23, 2007, which is hereby incorporated by reference. In some embodiments, the IROX coating can have a rice grain surface morphology, such as the morphologies similar to those shown in FIGS. 3A and 3B. In other embodiments, the IROX coating can have a mud-cracked SEM morphology similar to the structures shown in FIGS. 3C and 3D.

The IROX coating can have capacitance of at least 0.006 Coul/cm² when scanned in a cyclic voltammetry test at a rate of no more than 50 mV/s. The scan can be between −1.05 V to 1.2 V, between −0.6V and 1 V or between −0.3 V to 1 V. The cyclic voltammetry test can be preformed in a phosphate buffered saline solution at room temperature. In some embodiments, the IROX coating can have a capacitance of at least 0.008 Coul/cm² when scanned in a Cyclic Voltammetry test at a rate of no more than 50 mV/s. Examples of Cyclic Voltammetry evaluations of different IROX samples are presented in the Examples section.

The IROX coating can be deposited by thermal decomposition, reactive laser ablation, physical vapor deposition (PVD) or electrochemical deposition, which can each include a host of process parameters that can impact the properties of the deposited IROX. Different deposition processes can result in different surface characteristics, different amounts of carbonyl oxygen groups (C═O bonds), and different possible σ:π ratios. For instance, thermal decomposition can have between 30 percent and 50 percent atomic carbon while PVD can have between 10 percent and 25 percent atomic carbon. The atomic carbon can form carbonyl oxygen groups within the IROX. Furthermore, different processes can produce different morphologies. For example, physical vapor deposition (PVD) can be used to produce the rice grain morphologies of FIGS. 3A and 3B, while thermal decomposition can be used to produce the cracked mud cake structures shown in FIGS. 3C and 3D. These processes can also be used to form other morphologies.

The operating parameters of the deposition system are selected to tune the σ:π ratio of the IROX and the mean valance state of the iridium atoms in the IROX, as well as other properties such as surface morphology. In particular, the power, total pressure, oxygen/argon ratio and sputter time can be adjusted to alter the σ:π ratio, the mean valance state, and other properties. In some embodiments, physical vapor deposition processes use oxygen gas and argon gas where the oxygen gas makes up greater than 90 percent of the gas in the PVD chamber. The oxygen partial pressure can be in the range of between 91% to 100% (e.g., 92%). In some embodiments, the power is at least 750 watts (e.g. between 850 watts and 1000 watts) and the total pressure is between 3 mTorr and 7 mTorr (e.g., between 4 mTorr and 6 mTorr). The deposition time controls the thickness of the IROX and the stacking of morphological features. In embodiments, the deposition time is between 5 minutes and 15 minutes (e.g. between 8 and 12 minutes). The overall thickness of the IROX can be between 50 nm and 500 nm (e.g. between 100 nm and 300 nm). Specific PVD processing conditions are discussed in the Examples section.

Thermal decomposition can also be used to produce the IROX coating. For example, thermal decomposition can produce a mud-cracked morphology coating shown in FIGS. 3C and 3D as seen using a scanning electron microscope. As discussed above, thermal decomposition can result in a higher percentage of atomic carbon than PVD processes, which can result in a higher percentage of carbonyl oxygen. In some embodiments, IROX is deposited using thermal decomposition temperature of between 200 and 250 degrees Celsius for less than about 10 minutes (e.g., between 1 minute and 5 minutes). The IROX can be annealed afterwards for less than one hour (e.g., between 10 minutes and 50 minutes). Thermal decomposition can also include steps of sand blasting and the application of a precursor solution prior to heating the iridium containing precursor material to form the iridium oxide. For example, U.S. Patent Application Publication Nos. 2006/0035026 and 2005/0131509 describe IROX deposition processes using thermal decomposition, which can be modified as discussed above, and are hereby incorporated by reference.

The stent can include (e.g., be manufactured from) a base metal, such as stainless steel (e.g., 316L, BioDur® 108 (UNS S29108)), and 304L stainless steel, and an alloy including stainless steel and 5-60% by weight of one or more radiopaque elements (e.g., Pt, Ir, Au, W) (PERSS®) as described in US-2003-0018380-A1, US-2002-0144757-A1, and US-2003-0077200-A1), Nitinol (a nickel-titanium alloy), cobalt alloys such as Elgiloy, L605 alloys, MP35N, titanium, titanium alloys (e.g., Ti-6Al-4V, Ti-50Ta, Ti-10Ir), platinum, platinum alloys, niobium, niobium alloys (e.g., Nb-1Zr) Co-28Cr-6Mo, tantalum, and tantalum alloys. Other examples of materials are described in commonly assigned U.S. application Ser. No. 10/672,891, filed Sep. 26, 2003; and U.S. application Ser. No. 11/035,316, filed Jan. 3, 2005, which are both hereby incorporated by reference. Other materials include elastic biocompatible metal such as a superelastic or pseudo-elastic metal alloy, as described, For example, in Schetsky, L. McDonald, “Shape Memory Alloys”, Encyclopedia of Chemical Technology (3rd ed.), John Wiley & Sons, 1982, vol. 20. pp. 726-736; and commonly assigned U.S. application Ser. No. 10/346,487, filed Jan. 17, 2003. Any stent described herein can be dyed or rendered radiopaque by addition of, e.g., radiopaque materials such as barium sulfate, platinum or gold, or by coating with a radiopaque material. For example, in some embodiments, the second coating 36 can be a radiopaque material.

The stent can be of a desired shape and size (e.g., coronary stents, aortic stents, peripheral vascular stents, gastrointestinal stents, urology stents, tracheal/bronchial stents, and neurology stents). Depending on the application, the stent can have a diameter of between, e.g., about 1 mm to about 46 mm. In certain embodiments, a coronary stent can have an expanded diameter of from about 2 mm to about 6 mm. In some embodiments, a peripheral stent can have an expanded diameter of from about 4 mm to about 24 mm. In certain embodiments, a gastrointestinal and/or urology stent can have an expanded diameter of from about 6 mm to about 30 mm. In some embodiments, a neurology stent can have an expanded diameter of from about 1 mm to about 12 mm. An Abdominal Aortic Aneurysm (AAA) stent and a Thoracic Aortic Aneurysm (TAA) stent can have a diameter from about 20 mm to about 46 mm. The stent can be balloon-expandable, self-expandable, or a combination of both (e.g., U.S. Pat. No. 6,290,721). The ceramics can be used with other endoprostheses or medical implants, such as catheters, guide wires, and filters.

The stent 20 can, in some embodiments, include a second coating over the IROX. The second coating can be a polymer. In some embodiments, the second coating can be a drug eluting coating. Suitable drug eluting polymers may be hydrophilic or hydrophobic. Suitable polymers include, for example, polycarboxylic acids, cellulosic polymers, including cellulose acetate and cellulose nitrate, gelatin, polyvinylpyrrolidone, cross-linked polyvinylpyrrolidone, polyanhydrides including maleic anhydride polymers, polyamides, polyvinyl alcohols, copolymers of vinyl monomers such as EVA, polyvinyl ethers, polyvinyl aromatics such as polystyrene and copolymers thereof with other vinyl monomers such as isobutylene, isoprene and butadiene, for example, styrene-isobutylene-styrene (SIBS), styrene-isoprene-styrene (SIS) copolymers, styrene-butadiene-styrene (SBS) copolymers, polyethylene oxides, glycosaminoglycans, polysaccharides, polyesters including polyethylene terephthalate, polyacrylamides, polyethers, polyether sulfone, polycarbonate, polyalkylenes including polypropylene, polyethylene and high molecular weight polyethylene, halogenerated polyalkylenes including polytetrafluoroethylene, natural and synthetic rubbers including polyisoprene, polybutadiene, polyisobutylene and copolymers thereof with other vinyl monomers such as styrene, polyurethanes, polyorthoesters, proteins, polypeptides, silicones, siloxane polymers, polylactic acid, polyglycolic acid, polycaprolactone, polyhydroxybutyrate valerate and blends and copolymers thereof as well as other biodegradable, bioabsorbable and biostable polymers and copolymers. Coatings from polymer dispersions such as polyurethane dispersions (BAYHDROL®, etc.) and acrylic latex dispersions are also within the scope of the present invention. The polymer may be a protein polymer, fibrin, collagen and derivatives thereof, polysaccharides such as celluloses, starches, dextrans, alginates and derivatives of these polysaccharides, an extracellular matrix component, hyaluronic acid, or another biologic agent or a suitable mixture of any of these, for example. In one embodiment, the preferred polymer is polyacrylic acid, available as HYDROPLUS® (Boston Scientific Corporation, Natick, Mass.), and described in U.S. Pat. No. 5,091,205, the disclosure of which is hereby incorporated herein by reference. U.S. Pat. No. 5,091,205 describes medical implants coated with one or more polyisocyanates such that the devices become instantly lubricious when exposed to body fluids. In another preferred embodiment of the invention, the polymer is a copolymer of polylactic acid and polycaprolactone. Suitable polymers are discussed in U.S. Publication No. 2006/0038027. In embodiments, the polymer is capable of absorbing a substantial amount of drug solution. When applied as a coating on a medical implant in accordance with the present invention, the dry polymer is typically on the order of from about 1 to about 50 microns thick. Very thin polymer coatings, e.g., of about 0.2-0.3 microns and much thicker coatings, e.g., more than 10 microns, are also possible. Multiple layers of polymer coating can be provided. Such multiple layers are of the same or different polymer materials.

The terms “therapeutic agent”, “pharmaceutically active agent”, “pharmaceutically active material”, “pharmaceutically active ingredient”, “drug” and other related terms may be used interchangeably herein and include, but are not limited to, small organic molecules, peptides, oligopeptides, proteins, nucleic acids, oligonucleotides, genetic therapeutic agents, non-genetic therapeutic agents, vectors for delivery of genetic therapeutic agents, cells, and therapeutic agents identified as candidates for vascular treatment regimens, For example, as agents that reduce or inhibit restenosis. By small organic molecule it is meant an organic molecule having 50 or fewer carbon atoms, and fewer than 100 non-hydrogen atoms in total.

Exemplary therapeutic agents include, e.g., anti-thrombogenic agents (e.g., heparin); anti-proliferative/anti-mitotic agents (e.g., paclitaxel, 5-fluorouracil, cisplatin, vinblastine, vincristine, inhibitors of smooth muscle cell proliferation (e.g., monoclonal antibodies), and thymidine kinase inhibitors); antioxidants; anti-inflammatory agents (e.g., dexamethasone, prednisolone, corticosterone); anesthetic agents (e.g., lidocaine, bupivacaine and ropivacaine); anti-coagulants; antibiotics (e.g., erythromycin, triclosan, cephalosporins, and aminoglycosides); agents that stimulate endothelial cell growth and/or attachment. Therapeutic agents can be nonionic, or they can be anionic and/or cationic in nature. Therapeutic agents can be used singularly, or in combination. Preferred therapeutic agents include inhibitors of restenosis (e.g., paclitaxel), anti-proliferative agents (e.g., cisplatin), and antibiotics (e.g., erythromycin). Additional examples of therapeutic agents are described in U.S. Published Patent Application No. 2005/0216074. Polymers for drug elution coatings are also disclosed in U.S. Published Patent Application No. 2005/019265A. A functional molecule, e.g. an organic, drug, polymer, protein, DNA, and similar material can be incorporated into groves, pits, void spaces, and other features of the ceramic.

The stents described herein can be configured for vascular, e.g. coronary and peripheral vasculature or non-vascular lumens. For example, they can be configured for use in the esophagus or the prostate. Other lumens include biliary lumens, hepatic lumens, pancreatic lumens, uretheral lumens and ureteral lumens.

IROX can also be used in other medical implants. IROX can improve the biocompatibility of a medical implant. Medical implants other than stents include orthopedic implants; bioscaffolding; bone screws; aneurism coils; heart and brain pacemakers, neural stimulators, and other endoprostheses such as covered stents and stent-grafts. IROX having a σ:π ratio greater than 1.3 and/or a mean valance state of the iridium in the IROX of less than 3.4+ can improve the catalytic efficiency of the IROX. The IROX can be useful in heart and brain pacemakers and as neural stimulators as a coating for electrodes because a surface capacitance of greater than 0.006 Coul/cm² can reduce polarization losses of the voltage waveform for a constant current pulse used to stimulate the heart or brain. This can result in improved charge injection properties for the electrode. This can also reduce polarization loses at the electrode's surface.

EXAMPLES

Three IROX layers on stents were deposited in PVD processes using the following process conditions shown in Table I.

TABLE I Process I Process II Process III Oxygen Gas to 40% 33% 92% Argon Gas Ratio Total Pressure 10 mTorr 10 mTorr 4.7 mTorr Magnetron Power 300 Watts 100 Watts 890 Watts (½ cathode) (Current controlled: 1.5 Amps) Deposition Time 3 minutes 3.75 minutes 10 minutes PVD apparatus Cylindrical Cylindrical Planer Magnetron Magnetron Magnetron

FIG. 8A depicts a cylindrical magnetron. The cylindrical magnetron has a cylindrical iridium target and includes a 20 centimeter diameter chamber. The cylindrical magnetron can be an inverted cylindrical physical vapor deposition as described in Siegfried et al., Society of Vacuum Coaters, 39^(th) Annual Technical Conference Proceedings (1996), p. 97; Glocker et al., Society of Vacuum Coaters, 43^(rd) Annual Technical Conference Proceedings-Denver, Apr. 15-20, 2000, p. 81; and SVC: Society of Vacuum Coatings: C-103, An Introduction to Physical Vapor Deposition (PVD) Processes and C-248—Sputter Deposition in Manufacturing, available from SVC 71 Pinion Hill, Nebr., Albuquerque, N. Mex. 87122-6726. A suitable cathode system is the Model 514, available from Isoflux, Inc., Rochester, N.Y. FIG. 8B depicts a planar magnetron having a planar target and includes a 45 centimeter diameter chamber. The planar magnetron can have pulsed bias capabilities. Other sputtering techniques include closed loop cathode magnetron sputtering. Pulsed laser deposition is described in application U.S. Ser. No. 11/752,735, filed May 23, 2007. Formation of IROX is also described in Cho et al., Jpn. J. Appl. Phys. 36(I)3B: 1722-1727 (1997), and Wessling et al., J. Micromech. Microeng. 16:5142-5148 (2006).

A comparison of IROX samples produced by PVD processes I, II, and III showed that the IROX samples show similar surface morphologies during visual inspection. At micro-level the IROX coatings produced by PVD are smooth, dense and featureless. At nano-level, however, the grain structural features of the coating are clearly evident. FIG. 3 A depicts the surface morphology of IROX samples produced using Process I and FIG. 3B depicts the surface morphology of IROX samples produced using Process III. All three processes produce rice grain surface morphologies. The surface morphology of IROX coatings were examined by Zeiss Supra 35VP Field Emission Scanning Electron Microscope (FESEM).

An electrochemical evaluation of samples produced by the three different processes, however, showed that IROX samples produced by process III have a larger capacitance than IROX samples produced by processes I and II, indicating that process III creates a larger surface area than processes I and II. In a series of tests, IROX samples produced by each of processes I, II, and III were scanned in different Cyclic Voltammetry experiments. The cyclic voltammetry tests were conducted in phosphate buffered saline solution at room temperature and ambient conditions, using stents coated with the IROX sample films as working electrodes, platinum wires as counter/auxiliary electrodes, and a saturated Ag/AgCl wire as a reference electrode. Solartron 1470E Multi-channel Potentiostat/Galvanostat coupled with 1455 Frequency Response Analyzer (FRA) and MultiStat/CorrWare/ZPlot software was used for data acquisition and analysis. Cyclic voltammograms were recorded using potential scan rates between 5 and 350 mV/s with potential windows ranging from −1.05 to 1.2 V and from −0.3 V to 1 V for the slower scans. The electrochemical impedance data were recorded using a 10 mV amplitude of sinusoidal voltage around the open circuit potential (OCP) over a frequency range from 100 KHz to 0.1 Hz. The capacitance values of iridium oxide sample films were derived by integration of cyclic voltammograms and by fitting the EIS data with the appropriate equivalent circuits.

FIG. 4A depicts the results of a cyclic voltammetry test of the samples scanned at a rate of 50 mV/s between −0.6 V to 1 V. The test of FIG. 4A showed an average capacitance of the IROX samples produced by process I of 0.0038844 Coul/cm², an average capacitance of the IROX samples produced by process II of 0.0048358 Coul/cm², and an average capacitance the IROX samples produced by process III of 0.0086621 Coul/cm². FIG. 4B depicts the results of a cyclic voltammetry test of the samples scanned at a rate of 50 mV/s between −0.5 V to 0.9 V. The test of FIG. 4B showed an average capacitance of the IROX samples produced by process I of 0.0036572 Coul/cm², an average capacitance of the IROX samples produced by process II of 0.004394 Coul/cm², and an average capacitance of the IROX samples produced by process III of 0.0082963 Coul/cm². The capacitance did not show dependence on the scan rate for the scans at rates of between 5 to 50 mV/s. FIGS. 4C and 4D depict the results of cyclic voltammetry tests of the samples at scan rates of 200 mV/s and 350 mV/s respectfully. FIGS. 4C and 4D also showed a larger charge capacity for IROX samples produced by process III. Scans faster than 50 mV/s, however, showed smaller capacitance values, which could be due to the faster scan rates resulting in less probing of deeper layers.

An Auger Electron Spectroscopy (AES) analysis of the IROX samples was also preformed. A PHI Model 680 Field Emission Nanoprobe was used for data acquisition and analysis. Auger electrons were excited with a primary electron beam voltage of 10 keV, an absorbed beam current of 8 nA at 55° sample tilt. Common carbon contamination, evident from the C (KLL) signal at 272 eV were at low levels of ˜15% at. The detected nitrogen impurity were at trace levels. The appearance of intense Auger O KL_(2,3)L_(2,3) signal at 512 eV in Iridium Auger signals, indicates the presence of oxidized iridium. The Auger spectra for each sample, however, indicated an 0 to Ir ratio of ˜1, which suggests that the oxides are reduced due to the electron beam damage due to the AES process. Accordingly, AES spectra can only provide a qualitative estimate of the elemental atomic composition of IROX.

The composition of IROX, however, can be determined by x-ray photoelectron spectroscopy (XPS). After argon sputtering, chemical states and elemental composition analysis by X-ray photoelectron spectroscopy of each IROX surface was conducted using PHI Quantera scanning XPS, equipped with a monochromatic Al Kα (1486.6 eV) X-ray source, at a base pressure bellow 10⁻⁹ mbar. The binding energy (BE) scale was calibrated relative to the BE of C 1s. The analyzed areas for survey and high resolution spectra were 300 and 150 μm respectively. Survey spectra were acquired at a detection angle of 45°. High resolution core level spectra were collected at pass energy of 55 eV. Least-square curve-fitting of the Ir 4f, O 1s and C 1s spectra was performed based on the summed 90%/10% Gaussian-Lorentzian functions with a Shirley background subtraction. The atomic surface composition of IROX coatings was calculated from the area of core level photoelectron peaks corrected by sensitivity factors of the respective elements from the PHI MultiPak software package. The relative concentration of the functional groups was determined from the areas under the corresponding resolved components with respect to the total core level photoemission. Elemental surface composition was also confirmed by the Auger Electron Spectroscopy (AES).

FIG. 5A shows the Iridium 4f core level spectra for samples of IROX produced by each process. The Iridium 4f core level spectra does not allow for a significant distinction between the IROX samples from each process, even when deconvoluted. FIG. 5B, however, shows the Oxygen 1s core level spectra for samples of IROX produced by each of processes I, II, and III. As shown, each process produces a different Oxygen 1s core level spectra shapes. When deconvoluted, as shown in FIGS. 6A-6C, the relative amounts of oxygen σ (single) bonds can be determined by the size of the peak 62 and the relative amounts of oxygen π (double) bonds can be determined by the size of peak 64. FIG. 6A depicts the deconvoluted Oxygen 1s core level spectra for IROX produced by process I. FIG. 6B depicts the deconvoluted Oxygen 1s core level spectra for IROX produced by process II. FIG. 6C depicts the deconvoluted Oxygen 1s core level spectra for IROX produced by process III. As shown, IROX produced by process III has a smaller oxygen π bond peak 64.

The Oxygen 1s core level spectra, however, does not always allow for the calculation of the σ:π ratio due to the presence of carbonyl oxygen in the IROX. As discussed above, many of the IROX deposition processes can incorporate between 10 and 50 percent atomic carbon and produce non-trace amounts of carbonyl oxygen. It is possible, however, to use Carbon is XPS core level spectra data to determine amount of oxygen-carbon bonds present due to the presence of carbonyl oxygen. FIGS. 7A-7C depict the deconvoluted Carbon 1s core level data for IROX samples produced by process I-III respectively. Peak 72 of the deconvoluted data represents the relative amount of carbon-oxygen π bonds. The amount of carbon-oxygen π bonds can then be subtracted from the 1s XPS σ oxygen bond value to determine the relative number of Ir—O σ bonds to the number of Ir═O π bonds.

TABLE 2 O functional group % at O in O in Ir—O σ C O Ir C functional group % at Ir═O π bonds + O in O in Ir—O σ Sample % at % at % at C—C C—O C═O bonds C═O bonds C—O Process I 11.3 63.0 25.7 6.8 2.9 1.6 35.3 20.2 18.6 7.6 20.6-1.6 Process I 10.4 62.8 26.8 6.3 2.6 1.46 35.2 20.1 18.6 7.5  20.1-1.46 Process I 13.0 60.8 26.3 8.97 2.7 1.3 35.9 18.8 17.5 6.1 18.8-1.3 Process I mean 11.6 1.45 19.7 18.2 Process II 14.2 61.2 24.6 9.4 3.5 1.3 33.05 20.8 19.5 7.3 Process II 14.2 61.0 24.7 8.1 4.4 1.7 32.3 20.7 19 7.9 Process II 17.6 58.9 23.5 11.4 4.2 1.9 30.6 21.2 19.3 7.1 Process II mean 15.3 1.63 20.9 19.3 Process III 15.2 61.6 23.2 10.3 3.0 1.8 18.5 33.9 32.1 9.2 Process III 19.0 59.0 22.0 12.2 4.4 2.5 20.6 29.5 27 8.8 Process III 18.7 59.0 22.4 12.2 4.3 2.1 21.2 29.5 27.4 8.3 Process III mean 17.6 2.1 30.97 28.83

TABLE 3 O in Ir═O O in Ir—O C O Ir O in C—O O in IROX Σ O in π bonds σ bonds Sample % at % at % at % at % at IROX/Ir % at % at O_(IrOx)/O_(IrOz) Process I 11.3 63.0 25.7 4.5 58.5 2.276 35.3 18.6 1.898 Process I 10.4 62.8 26.8 4.1 58.7 2.19 35.2 18.6 1.892 Process I 13.0 60.8 26.3 4 56.8 2.16 35.9 17.5 2.05 Process I mean 26.3 2.21 35.47 18.23 1.95 Process II 14.2 61.2 24.6 4.8 56.4 2.293 33.05 19.5 1.695 Process II 14.2 61.0 24.7 6.1 54.9 2.223 32.3 19 1.7 Process II 17.6 58.9 23.5 6.1 52.8 2.247 30.6 19.3 1.585 Process II mean 24.3 2.25 32.0 19.3 1.66 Process III 15.2 61.6 23.2 4.8 56.8 2.448 18.5 32.1 0.576 Process III 19.0 59.0 22.0 6.9 52.1 2.368 20.6 27 0.763 Process III 18.7 59.0 22.4 6.4 52.6 2.348 21.2 27.4 0.774 Process III mean 22.5 2.39 20.1 28.83 0.704

The average chemical structure of IROX is determined using the formula of IrO_(x)(OH)_(y), where x and y are calculated from the data in Tables 2 and 3. The σ:π ratio is equal to y divided by x. The mean valance state of the IROX is 2x+y. The value of x is calculated to be the atomic percent of oxygen in Ir═O π bonds divided by the atomic percent of iridium. As discussed above, the amount of oxygen in Ir═O π bonds is determined by a deconvolution of the Oxygen 1s core level spectra data. The amount of iridium is calculated by XPS data. The value of y is calculated by calculating the ratio of oxygen in IROX to iridium and subtracting the value of x. The amount of oxygen in IROX is calculated by from detecting the amount of total oxygen from the Oxygen 1s core level spectra data and subtracting the amount of oxygen due to the presence of carbon-oxygen bonds from the Carbon is XPS core level spectra data. The y value is calculated by subtracting the x value from the ratio to the oxygen in IROX to iridium due to possible errors in the deconvolution of the XPS data for determining the amount of oxygen in Ir—O σ bonds.

For example, process I is determined to have an average chemical structure of IrO_(1.35)(OH)_(0.86), a σ:π ratio of about 0.64, and a mean iridium valance state of about 3.56. The value of x for process I is determined by dividing the atomic percent of oxygen in Ir═O π bonds, which is 35.47, by the atomic percentage of iridium, which is 26.3, to yield an x value of about 1.35. The value for y for process I is determined by dividing the sum of oxygen in IROX, which is an average of 58, divided by the atomic percentage of iridium, which is 26.3, to yield a value of 2.21 and subtracting the value of x, which is 1.35, to yield a y value of 0.86. The σ:π ratio of about 0.64 is calculated by dividing the y value by the x value and the mean iridium valance state is calculated by multiplying the x value by two and adding the y value. Process II is determined to have an average chemical structure of IrO_(1.32)(OH)_(0.93), a σ:π ratio of about 0.70, and a mean iridium valance state of about 3.57. Process III is determined to have an average chemical structure of IrO_(0.89)(OH)_(1.5), a σ:π ratio of about 1.69, and a mean iridium valance state of about 3.28. The different samples of IROX produced by process III shown in Tables 2 and 3 have the following formulas, σ:π ratios, and mean valance states: IrO_(0.80) (OH)_(1.65), about 2.06, and about 3.25; IrO_(0.94)(OH)_(1.43), about 1.52, and about 3.31; and IrO_(0.95)(OH)_(1.40), about 1.47, and about 3.30.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other embodiments are within the scope of the following claims. 

1. A medical implant comprising iridium oxide, the iridium oxide having a plurality of Ir—O σ bonds and a plurality of Ir═O π bonds, the iridium oxide having a ratio of the Ir—O σ bonds to the Ir═O π bonds that is greater than 1.3.
 2. The medical implant of claim 1, wherein the ratio is greater than 1.45.
 3. The medical implant of claim 1, wherein the ratio is less than
 3. 4. The medical implant of claim 1, wherein the ratio is between 1.6 and 2.1.
 5. The medical implant of claim 1, wherein the iridium oxide has a plurality of Ir atoms having a mean valance state of less than 3.4+.
 6. The medical implant of claim 1, wherein the iridium oxide comprises a morphology of defined grains with an aspect ratio of about 5:1 or more.
 7. The medical implant of claim 1, wherein the iridium oxide exhibits a charge of at least 0.0060 Coul/cm² in a cyclic voltammetry potentiodynamic electrochemical measurement having a sweep rate no more than 50 mV/s.
 8. The medical implant of claim 1, wherein the iridium oxide defines an outer surface of the medical implant.
 9. The medical implant of claim 1, further comprising a base metal, the iridium oxide coating the base metal.
 10. The medical implant of claim 9, wherein the base metal is selected from the group consisting of stainless steels, stainless steels enhanced with radiopaque elements, nickel-titanium alloys, cobalt alloys, titanium and titanium alloys, platinum and platinum alloys, niobium and niobium alloys, tantalum and tantalum alloys, and combinations thereof.
 11. The medical implant of claim 1, wherein the medical implant comprises an electrode, the electrode comprising the iridium oxide.
 12. The medical implant of claim 1, wherein the medical implant is an endoprosthesis.
 13. The medical implant of claim 12, wherein the endoprosthesis is a stent.
 14. A medical implant comprising iridium oxide, the iridium oxide having a plurality of Ir atoms having a mean valance state of less than 3.4+.
 15. The medical implant of claim 14, wherein the Ir atoms having a mean valance state of between 3.20+ and 3.35+.
 16. The medical implant of claim 14, wherein the iridium oxide has a plurality of Ir—O σ bonds and a plurality of Ir═O π bonds, the iridium oxide having a ratio of the Ir—O σ bonds to the Ir═O π bonds that is between 1.45 and 3.0.
 17. The medical implant of claim 14, further comprising a base metal, the iridium oxide coating the base metal and defining an outer surface of the medical implant, the base metal being selected from the group consisting of stainless steels, stainless steels enhanced with radiopaque elements, nickel-titanium alloys, cobalt alloys, titanium and titanium alloys, platinum and platinum alloys, niobium and niobium alloys, tantalum and tantalum alloys, and combinations thereof.
 18. The medical implant of claim 14, wherein the medical implant comprises an electrode, the electrode comprising the iridium oxide.
 19. The medical implant of claim 14, wherein the medical implant is an endoprosthesis.
 20. A method for making a medical implant comprising: placing a medical implant or precursor thereof in a physical vapor deposition processing chamber; and depositing iridium oxide on the medical implant or precursor thereof using a power of at least 750 watts, and a total pressure of between 3 mTorr and 7 mTorr. 